As will be apparent from our prior applications mentioned above and the developments in the transport and storage of nuclear wastes referred to by the art of record in these applications, it is recognized that the storage of radioactive materials, such as spent nuclear fuel rods or other materials, can be effected with canister receptacles or vessels, which are sealed after the nuclear waste is introduced and which have radiation-shielding properties as a result of the wall thickness of the vessel body and/or radiation absorbing characteristics of the material, from which the body is composed.
For example, the body may be made of a material having a high neutron cross section or materials with a high neutron cross section can be incorporated in the body. Alternatively, or in addition, passages or spaces may be formed in the body and gamma-ray or neutron absorptive or moderating materials can be introduced.
Obviously the vessel must have excellent structural integrity as well as a capacity to act as a radiation shielding material.
The canister basically comprises a chamber-forming body or receptacle, advantageously upwardly open, with a relatively thick vertical wall and a closed bottom and a cover for the upper end or mouth of the chamber.
The vertical walls are usually comprised in one piece with the bottom of cast iron, preferably spherolytic cast iron, or cast steel, while a recess is formed in the upper end of the body so that the cover structure can be recessed therein with a plug-like fit and an upper surface flush with the upper surface of the body.
The earlier systems also provided for sealing the cover to the body and even monitoring the state of the seal to ascertain whether any leakage may have occurred.
Such monitoring is relatively simple because, in addition to the radioactive materials filling the chamber, the latter receives a control gas blanket and any failure of the seal can be detected by monitoring the composition of this control gas or detecting the presence thereof.
It has also been noted that the cast iron or steel alloy can include radiation-absorbing alloying components and/or radiation-absorbing inclusions.
The term "radiation shielding" as used to describe a canister or vessel, thus defines a vessel whose thickness at any point (wall, cover or bottom) is sufficient to prevent escape of radiation, whether the radiation be of the gamma or neutron type and which also is able to withstand the mechanical stresses to which the vessel may be subjected in handling the in-transport or the like, both during normal manipulations and in the event of a disaster, such as a crash of a transporting vehicle.
The most general application utilizes flange-type covers which are bolted in place to the body, and it has been recognized that these systems may be problematic because the seal is maintained only as long as the bolts are tight or intact.
In transport accidents, however, the bolted flange receptacles run the risk of shearing or loosening of the bolts and release of radioactive materials into the environment.
Thus it is possible to "sniff out" a failure of the seal by monitoring the presence of the control gas outside of the edge of the cover and utilizing an appropriate gas detector or analyzer.
If a failure of the seal is observed, components of the radioactive waste or gaseous substances, which are formed by radioactive decay of the waste, can be found in the gas and may wander out of the receptacle.
Repair of the defective seam or seal is not a simple matter because removal of the cover can result in a serious contamination of the surrounding space by release of radioactive materials from the interior. The repair thus can only be carried out in a so-called hot-cell at considerable cost.